Exchange users and Send as

Any Exchange 07 gurus out there? I have created a user on my Exchange 07 server who I want to be able to send email from my mailbox on behalf of - it's basically an alias for me. Email addressed to that user is successfully landing in both inboxes: mine and the other user's - but despite having enabled Send As permission, I can't seem to reply as from the other user (I'm the administrator). I'd also like to be able to switch between inboxes in OWA without having to log out and back in. Any suggestions gratefully received, thank you.